Screw machines of this type are used to prepare plastics materials and are, for example, known from EP 0 522 390 B2. The plastics material to be prepared is subjected to shear flows in these screw machines and thus melted, mixed and homogenized. To avoid thermally damaging shear peaks, the kneading discs of EP 0 522 390 B2 have an enlarged passage cross section produced by material removal in the axial direction. The drawback is that plastics materials with high-molecular particles can only be inadequately homogenized with the forced shear flows, so so-called spots—also called “white spots”—remain in the materials.